Separating volatile components from a solution by distillation is known, and several types of distillation are well known. Distillation takes advantage of differences in the property of volatility, or boiling point, which refers to the tendency of a substance to evaporate. Substances of higher volatility evaporate more easily, and a highly volatile substance has a low boiling point. Conversely, a substance of low volatility has a high boiling point. Another quantitative measure of the volatility of a substance is its vapor pressure.
Standard distillation is typically performed in a distillation column, which includes a series of vertically spaced plates. A feed stream enters the column at a mid-point, dividing the column into two sections; the top section being called the rectification section, and the bottom section being called the stripping section. Condensation and vaporization occur on each plate, causing lower boiling point components to rise to the top of the column and higher boiling point components to fall to the bottom. A re-boiler is located at the base of the column to add thermal energy. The “bottoms” product is removed from the base of the column. A condenser is located at the top of the column to condense the product emanating from the top of the column, which is called the distillate. A reflux pump is used to maintain flow in the rectification section of the column by pumping a portion of the distillate back into the column.
In any solution, the vapor above the solution will be a combination of all of the volatile components in the solution, but it will have more of the component that evaporates more easily (i.e., the more volatile substance). In other words, the vapor above a 50/50 mixture of two different liquids will have more than 50% of the component having a higher volatility, and the gases from the less volatile liquid would constitute less than 50%. If one were to collect the gases above the mixture and condense them (turn them back to liquid), the condensed liquid would no longer be a 50/50 mixture. The condensed liquid would be “enriched” with the more-volatile liquid. Enriched means the condensed liquid would contain more than 50% of the more-volatile liquid and less than 50% of the less-volatile liquid. In a normal distillation process, the original mixture is heated to increase the rate at which the liquids evaporate so that the separation occurs more quickly.
For example, standard distillation can separate pure water from a solution that contains table salt (NaCl) and water. The “normal” boiling point of water is 100° C., and the boiling point for NaCl is 1413° C. Because water has the lower boiling point, it is more volatile and evaporates more easily than does NaCl. In distillation of salt water, the mixture is heated until it boils. The water vaporizes but the NaCl does not to any measurable extent. The water vapor leaves the original container and is condensed back into liquid water. Since the NaCl never vaporized, it remained in the original container, and the condensed liquid is essentially pure water.
Distillation may also be used to separate the components of a mixture of ethanol and water. Because ethanol is more volatile than water, it evaporates more easily. The condensed liquid, after distillation of a mixture of ethanol and water is enriched in ethanol. The boiling point of ethanol is 78° C., and the boiling point of water is 100° C. Because these boiling points are so close to each other, several distillations are required to achieve maximum separation. Due to the fundamental natures of ethanol and water, 100% separation is not possible by simple distillation because an azeotrope forms. The maximum separation of ethanol and water by simple distillation is about 95% ethanol and 5% water solution.
To get best separation distillation and condensation must be performed a number of times. By using a technique called fractional distillation, many distillations can occur at the same time. In a common oil refinery, this occurs in a fractionating tower, where the temperature at the top is lower than the temperature at the bottom. As a result, the gas fraction comes out at the top of the tower followed by gasoline, kerosene; diesel fuel, lubricating oil, and, finally, the asphalts at the bottom.
Azeotrope distillation utilizes or forms an azeotrope by adding a pure liquid to a liquid mixture to enhance the separability of two or more of the components with similar boiling points. Azeotropic distillation separates liquid mixtures according to their vapor pressures. A liquid mixture having components X and Y, where X and Y cannot be separated by the simple distillation described above, a third liquid Z can be added to create an azeotrope. Component Z, the entrainer, is chosen because it will form a relationship with component X such that they have the same proportions in both the liquid and vapor phases. This means that at a given temperature both Z and X will remain in the liquid phase and all of component Y will vaporize or Y will remain in the liquid phase and a portion of X and Z will vaporize. Maximum boiling point azeotropes are removed as bottoms and minimum boiling point azeotropes are removed as distillates.
In a typical azeotropic distillation procedure, a third component, such as benzene, isopropyl ether or cyclohexane, is added to an azeotropic mixture, such as ethyl alcohol/water, to form a ternary azeotrope. Since the ternary azeotrope is richer in water than the binary ethyl alcohol/water azeotrope, water is carried over the top of the column. The ternary azeotrope, when condensed, forms two phases. The organic phase is refluxed to the column while the aqueous phase is discharged to a third column for recovery of the entraining agent. Certain azeotropes such as the n-butanol/water mixture can be separated in a two-column system without the use of a third component. When condensed and decanted, this type of azeotrope forms two phases. The organic phase is fed back to the primary column and the butanol is recovered from the bottom of the still. The aqueous phase, meanwhile, is charged to the second column with the water being taken from the column bottom. The vapor streams from the top of both columns are condensed, and the condensates run to a common decanter.
The combination of chemical reaction with distillation in only one unit is called reactive distillation. Reactive distillation combines reactors and distillation units when reversible or consecutive chemical reactions occur. In reactions where the amount of desired product formed is limited by an equilibrium equation, such as esterification and ester hydrolysis reactions, continuously removing the product from the reaction by distillation yields an extent of reaction that far exceeds the amount of product that would be obtained by a batch reaction followed by distillation. Extractive distillation is somewhat similar to azeotropic distillation in that it is designed to perform the same type of task. In azeotropic distillation, the azeotrope is broken by carrying over a ternary azeotrope at the top of the column. In extractive distillation, a higher boiling compound is added and the solvent to be recovered is pulled down the column and removed as the bottom product. A further distillation step is then required to separate the solvent from the entraining agent. This is the process by which an agent is added to modify the relative volatility between the key components without forming an azeotrope.
In distillation terminology, “stripping” refers to the removal of a volatile component from a less volatile substance. For example in an ethyl alcohol/water system, stripping is done in the column below the feed point, where the alcohol enters at about 10% by weight and the resulting liquid from the column base contains less than 0.02% alcohol by weight. This is known as the stripping section of the column. This technique does not increase the concentration of the more volatile component, but rather decreases its concentration in the less volatile component. A stripping column also can be used when a liquid such as water contaminated by toluene cannot be discharged to sewer. For this pure stripping duty, the toluene is removed within the column, while vapor from the top is decanted for residual toluene recovery and refluxing of the aqueous phase.
The term “steam stripping” can be applied to any system where rising steam vapors in a column strip out the volatile components in the liquid. In particular, the term is applied to systems where steam is used to strip out partially miscible organic chemicals, even though the organic chemicals have boiling points above water. For example, toluene, which has a boiling point of 110° C., can be stripped out of water with steam. The low solubility of toluene in water changes the activity coefficient, and the toluene can be stripped off as the water/toluene azeotrope.
For water miscible and water immiscible high volatile compounds, the process is a relatively straight forward distillation system. For many of the systems, vapor liquid equilibrium data are available in the literature and in the many process simulation software programs. Steam stripping can also be used to remove low-volatile components when the components have low miscibility with water. Those compounds can all be effectively removed from water by steam stripping, even though they have a lower volatility than water. This technique has been used for many years, particularly in the petroleum industry, where the presence of steam with low miscibility organics has allowed for high boiling compounds to be distilled at lower temperatures. Due to the low solubility in water, the activity coefficient is greatly increased and the compound forms a low boiling point azeotrope with water. The lower the solubility, the higher the enhancement of the activity coefficient. A general rule is that the ease of stripping of any VOC is directly proportional to its volatility, and since, in practice, it enables some high boiling toxic compounds such as PCBs to be removed by steam stripping. High boiling, fully water miscible compounds cannot be removed by steam stripping. In these cases the water can be removed as distillate from a distillation process.
A problem arises with separating components by known distillation processes when the target component to be recovered is of low volatility and yet tends to degrade at the higher temperatures required for its vaporization at any significant rate. This problem is accentuated further when the starting solution contains contaminants of even lower volatility, such as the numerous salts that are ordinarily found in recovered water/ethylene glycol solutions. For example, in the common separation of ethylene glycol from water, the water is first removed by distillation at one temperature, and the ethylene glycol is separated from the contaminating salts at a higher temperature. Because the ethylene glycol degrades at the higher temperature, the ethylene glycol obtained by this process is degraded and of lesser value. Another example is the separation of amines from non-volatile contaminants, such as the amines used for removing carbon dioxide from natural gas. These amines accumulate contaminants but separation by known distillation techniques degrades the amines.
Accordingly, it is an object of this invention to provide a process that allows separation of a target component from a solution by distillation at lower temperatures that do not degrade the target component.
Another object of the invention is to provide a method whereby a target component is removed from a mixture and contaminating components are easily separated from the evaporator bottoms by precipitation.